Process for the preparation of an olefin polymerization catalyst

ABSTRACT

An improved process for the preparation of a catalyst for the polymerization of ethylene, and an improved process for preparing an ethylene polymer therewith. An improved supported chromium-containing catalyst is prepared by reacting (1) a chromium chelate of a 1,3-diketo compound and (2) a vanadium chelate or a vanadyl chelate of a 1,3-diketo compound, separately or jointly with (3) an organometallic compound of an element from Group II or III of the periodic system, jointly contacting the resulting reaction products of (1) and (2) with (3) with an inert inorganic supporting material so as to deposit such reaction products thereon, whereafter the supporting material containing the reaction products is heated in a non-reducing atmosphere at a temperature of between 200° and 1200° C. The 1,3-diketo compounds of (1) and (2) are the same or different, and have the formula ##STR1## wherein R 1 , R 2  and R 3  are the same or different, R 1  and R 3  being an alkyl group of 1-10 carbon atoms, and R 2  being selected from the group consisting of an alkyl group with 1-10 carbon atoms, and a hydrogen atom. The catalyst so prepared can be used either alone, or together with an organo-metallic compound of an element of Group II or III of the periodic system, in the polymerization of polyethylene or in the copolymerization of polyethylene with up to 15 mole percent of one or more α-alkenes having 3 to 15 carbon atoms.

BACKGROUND OF THE INVENTION

The present invention relates to an improved process for the preparationof supported chromium oxide-type catalysts used for the polymerizationof olefins, and to an improved process for the polymerization and/orcopolymerization of ethylene.

The polymerization of olefins, particularly of ethylene, with supportedchromium oxide-type catalysts, has been known for a long time. Suchpolymerization can be carried out as a "solution process", in which thepolymer is obtained as a solution in the solvent used for thepolymerization, such as is described in U.S. Pat. No. 2,825,721. Thepolymerization of ethylene using a supported chromium oxide-type ofcatalyst can also be carried out as a `slurry process`, in which theethylene polymer is precipitated, and is obtained as a slurry. Such amethod is described in U.S. Pat. Nos. 2,825,721 and 3,087,917. Solutionpolymerization processes are generally carried out at temperatures of atleast 120° C., whereas slurry polymerization processes are carried outat temperatures of about 110° C. at the most.

The molecular weight of the resulting ethylene polymer increases as thepolymerization temperature is decreased. Thus, with the relatively lowtemperature slurry polymerization processes, ethylene polymers havingvery low melt indices, i.e., very high molecular weights, are oftenobtained. There are many catalyst of the supported chromium oxide-typewhich are very suitable for use in higher temperature solutionpolymerization processes operating at temperatures of, for example, 140°C. or higher. However, in slurry polymerization processes, thesecatalysts yield only high molecular weight polymers.

It is known that the molecular weight of the polymer can be reduced byuse of molecular weight controllers. The most common molecular weightcontroller is hydrogen. However, many supported chromium oxide-catalystsshow very little sensitivity to hydrogen. Accordingly, to utilize thesecatalysts in slurry polymerization processes, large quantities ofhydrogen must be added to the monomer in order to obtain the desiredmolecular weights. Consequently, the effectiveness of the catalyst isstrongly reduced, lower yields are obtained and, moreover, largequantities of hydrogen must be recirculated with relatively smallquantities of monomer. Such methods are thus economically veryunattractive, and are often hardly feasible technically to carry out.

On the other hand, slurry polymerization processes have generally abetter efficiency than solution processes. For this reason, there is aneed for catalysts which are capable of making lower molecular weightethylene polymers in slurry polymerization processes, which catalystsare more sensitive to variations in temperature and to molecular weightcontrollers such as hydrogen. Moreover, there is a wide variation in thecorrelation between changes in polymerization temperature and resultingpolymer molecular weight from catalyst to catalyst. With many catalysts,the molecular weight shows very little variation with changes in thepolymerization temperature. In some prior art catalysts, however, it isstronger, and with these catalysts molecular weight can be betterinfluenced by the choice of the polymerization temperature. However, inthis respect they are not fully satisfactory, and there remains a needfor further improvement. In using these catalysts it is also verydesirable that limited quatities of hydrogen can bring about a definitelowering of the molecular weight of the resulting polymer.

Various modifications of supported chromium oxide-types of catalystshave been proposed in order to influence and control molecular weightsof ethylene polymers prepared therewith. Thus, it is described in U.K.Pat. No. 1,231,322, and in U.S. Pat. Nos. 3,812,058; 3,862,104;3,900,457; and 3,974,101, that the melt index of resulting ethylenepolymers is a function of the method used to prepare the silica support,and of the activation by heating of the supported chromium oxide. Thus,by making suitable choices among these preparative methods, polyethylenehaving higher melt indices can be prepared by means of a slurrypolymerization process.

It is an object of the present invention to provide an improved processfor preparing a supported chromium oxide-type of catalyst which can beeffectively utilized in slurry polymerization processes as well as insolution polymerization processes. It is a further objective of thisinvention to provide a chromium oxide-type catalyst having greatersensitivity to polymerization temperature and to molecular weightcontrollers such as hydrogen in affecting the molecular weight of thepolymer produced, as compared to known catalysts. It is furthermore anobjective of this invention to provide an improved process for thepolymerization of ethylene, or the copolymerization of ethylene with oneor more α-alkenes, utilizing the improved catalyst prepared inaccordance with this invention.

SUMMARY OF THE INVENTION

It has now been found that a catalyst can be obtained which can be usedfor both solution polymerization and slurry polymerization processes forpreparing polyethylene by reacting a chromium chelate of a 1,3-diketocompound, and a vanadium chelate or a vanadyl chelate of a 1,3-diketocompound, separately or jointly with organo-metallic compounds of anelement from Group II or III of the Periodic System in which hydrocarbylgroups with 1-20 carbon atoms are bound, via carbon atom, to therelative element. The resulting reaction products are thereuponcontacted with an inert inorganic supporting material in such manner asto deposit the reaction products thereon. The supporting material, withthe reaction products deposited thereon, is then heated in anon-reducing atmosphere at a temperature of between about 200° and 1200°C. The 1,3-diketo compounds of the chromium chelate and the vanadium orvanadyl chelate are the same or different, and have the formula ##STR2##wherein R₁, R₂, and R₃ are the same or different, R₁ and R₃ being analkyl group with 1-10 carbon atoms, and R₂ being either an alkyl group,having from 1-10 carbon atoms, or a hydrogen atom.

It is known in the prior art that a supported chromium oxide-type ofcatalyst for the polymerization of olefins can be prepared by bringingthe conversion product of a chromium-1,3-diketo compound and anorgano-metallic compound of an element from Group II or III of thePeriodic System into contact with an inert inorganic supporting materialwhereafter it is heated in a non-reducing atmosphere at a temperaturebetween 200° and 1200° C. Such a process is disclosed, in U.S. Pat. No.4,146,695 wherein such a supported chromium oxide component is utilizedtogether with an organo-metallic compound of an element from Group II orIII of the Periodic System, for the polymerization of ethylene, and forthe copolymerization of ethylene with up to 15 mole percent of one ormore α-alkenes having 3 to 5 carbon atoms.

It is also known from U.S. Pat. No. 3,635,840 that catalysts for thepolymerization of olefins can be prepared by contacting a previouslyformed supported chromium oxide with an organo-metallic compound and avanadium chelate or a vanadyl chelate of a 1,3-diketone, whereupon theresulting product obtained is heated in an oxygen-containing gas attemperatures of from 500° to 1500° F. (260° to 816° C.). According tothe example of said patent, vanadium acetylacetonate in a cyclohexanesolution was blended with triethylealuminum, and the resulting solutionused to impregnate a previously prepared chromium oxide catalyst. Theresulting chromium oxide catalyst so impregnated was thereupon heatedfor twenty-four hours at 1050° F. (566° C.). The catalyst so preparedwas then utilized for the solution polymerization of ethylene incyclohexane. Where the polymerization was carried out at a temperatureof above 293° F. (145° C.), the polyethylene thus formed had a meltindex of 0.17. By comparison, polyethylene prepared with this samecatalyst at a reactor temperature of 299° F. (148.3° C.), had a meltindex of 0.18.

It has now surprisingly been found that when carrying out thepolymerization of ethylene using a catalyst of the present invention,ethylene can be polymerized at a temperature of only about 100° C. underconditions otherwise similar to the above noted example, to produce apolyethylene having a melt index of about 0.2. Moreover, the temperaturesensitivity of the catalyst prepared in accordance with this inventionproves to be considerably greater than with the known catalyst. Thus,with the catalyst according to U.S. Pat. No. 3,635,840, a difference intemperature of 3.3° C. will result in a change in melt index of thepolyethylene prepared therewith of no more than 0.01. However, with thecatalyst of the present invention, the polyethylene melt index caneasily be varied to a considerable degree by only small variations inpolymerization temperature, as is shown by the examples, particularly byexamples 6, 7 and 8. Moreover the present catalysts give polyethyleneswith suitable melt indexes in the slurry polymerization temperaturerange, as shown by the examples 2-5. Moreover the catalysts of thisinvention also have good hydrogen sensitivity, so that the addition ofonly limited quantities of hydrogen will bring about a substantialreduction in the molecular weight of the polyethylene prepared.

Thus, when utilizing the catalyst prepared in accordance with thisinvention, it has been found that the slurry polymerization of ethylenecan effectively and advantageously be carried out at polymerizationtemperatures of between about 80° C. and 105° C., producing polyethylenehaving melt indeces in the range from about 0 to 4. It is furthermore aparticular advantage of these catalysts that they can be used in eithersolution or slurry types of polymerization processes.

The 1,3-diketo complexes of both chromium and of vanadium used in theinvention are chelates of these metals with 1,3-diketo compounds havingthe general formula ##STR3## wherein R₁ and R₃ are the same ordifferent, and each represent an alkyl group having 1-10 carbon atoms,and wherein R₂ may be either an alkyl group having 1-10 carbon atoms ora hydrogen atom.

Suitable 1,3-diketo compounds for use in this invention includeacetylacetone; hexane-2,4-dione; heptane-2,4 dione; octane-2,4-dione;octane-3,5-dione and homologues thereof, wherein R₂ is an alkyl groupwith 1 to 10 carbon atoms. Preferably, however, a 1,3-diketone is usedin which R₂ is a hydrogen atom, and acetylacetone is particularlypreferred.

Preferably, the chromium-1,3-diketo complex is thechromium(III)-acetylacetonate. Acetylacetonates of vanadium arepreferred as well, and may be V(acac)₂ ; V(acac)₃ ; VO(acac)₂ andVO(acac)₃, wherein "acac" represents the acetylacetone residue orradical. Similar compounds of other 1,3-diketones can also be used.However, good results are obtained with vanadium(III)acetylacetonate,and the use of this latter complex is preferred.

The 1,3-diketo compounds, preferably the acetylacetonates, are convertedwith an organo-metallic compound of an element from Group II or III ofthe periodic system, such as beryllium, magnesium, boron, aluminum orgallium. The organometallic compounds of transition elements from GroupsII and III are preferably compounds of the type (R₄)_(m) Me, wherein R₄represents a hydrocarbyl group, and "m" is either 2 or 3 depending uponthe valency of the element "Me". The hydrocarbyl groups of thesecompounds are preferably alkyl groups with 1-20 carbon atoms.

Aluminiumtrialkyls and magnesiumdialkyls have been found to beparticularly suitable as the organometallic compound used in thisinvention. The alkyl groups in the magnesium dialkyl preferably contain4 to 12 carbon atoms, and dibutylmagnesium and diisobutylmagnesium areparticularly preferred. Suitable organomagnesium compounds for use inaccordance with this invention also include diethylmagnesium,dipropylmagnesium, diisopropylmagnesium, diamylmagnesium,dihexylmagnesium, dioctylmagnesium, didecylmagnesium, anddidodecylmagnesium. Dicycloalkylmagnesium with the same or differentcycloalkyl groups having 3 to 12 carbon atoms, preferably 5 to 6 carbonatoms, is also suitable. Likewise, an alkyl and a cycloalkyl group canbe bound to the magnesium. Although alkyl- or cycloalkylmagnesiumcompounds are preferred, magnesium aryls can also be used, if desired,particularly diphenylmagnesium, but ditolyl- and dixylyl magnesium canbe used as well. The diarylmagnesium compounds are generally at mostpoorly soluble in aliphatic hydrocarbons and are, therefore, dissolvedin aromatic hydrocarbons.

The organomagnesium compounds can be prepared in any known manner, forinstance by the method as described in Organometallic Compounds, Vol. 1by G. E. Coates, M. L. H. Green, and K. Wade; and inOrganometallverbindungen by F. Runge. Particularly suitable solutions ofmagnesiumalkyls can be prepared in accordance with the method disclosedin the U.S. Pat. No. 3,737,393, that is herewith incorporated byreference.

The preferred aluminum compounds are aluminumtrialkyls. However, otherorganoaluminum compounds having the general formula of (R₅)₂ AlX,wherein R₅ represents an alkyl group with 1-10 carbon atoms, and X ahydrogen atom or an alkoxy group, can also be used. It is desirable,however, to minimize the occurrence of alkoxy groups. Furthermore,aluminum compounds as disclosed, for instance, in German"Auslegeschriften" Nos. 1,956,353; 1,183,084; 1,136,113, and 1,186,633,containing one or more groups derived from a diene, can also be used. Ofcourse, mixtures of these various organometallic compounds can be used.

The conversion of the diketo compounds in accordance with the inventionwith an organometallic compound of an element from Group II or III iscarried out in a solvent which is inert with respect to these compounds.Preferably, this conversion is carried out in a hydrocarbon solvent,most preferably in one or more linear or branched aliphatichydrocarbons, such as butane, pentane, hexane, heptane, octane, decane,or branched isomers thereof, or in a light petrol consisting mainly ofhexanes and/or heptanes, or in a heavier petrol. Higher linear orbranched saturated aliphatic hydrocarbons or mixtures thereof, can alsobe used as the solvent.

Chromium(III)acetylacetonate is more soluble in aromatic hydrocarbonsthan in aliphatic hydrocarbons, and it can be converted withmagnesiumdiaryls soluble in aromatic hydrocarbons. However, because ofthe cost of aromatic hydrocarbon solvents, and also due to the possiblehealth risks associated with aromatic hydrocarbon solvents, they aregenerally not used if suitable aliphatic and/or cycloaliphatic solventscan be used instead.

Because of their poor solubility in aliphatic and/or cycloaliphatichydrocarbons, only minor amounts of chromium(III)acetylacetonate and thevanadium acetylacetonates are dissolved, the rest remaining insuspension in the solvent. However, when an organomagnesium ororganoaluminum compound is added, the acetylacetonates will go at leastfor the major part into solution. This dissolution can be promoted bylightly heating the solution, for instance, at temperatures of fromabout 40° to 100° C. or, where low-boiling solvents are used, to theboiling point of the solvent. The heating many optinally be conductedunder pressure.

The acetylacetonates will color the hydrocarbon solvent lightly becausea small part dissolves therein. However, when the organomagnesium ororganoaluminum compound is added, dark colored solutions will be formed.Thus, the dark violet chromium(III)acetylacetonate will dissolve inlight petrol only to a small degree, resulting in a faintlyviolet-colored solution. However, when a magnesium or aluminumalkyl ispresent, considerable quantities of chromium(III)acetylacetonate will gointo solution, and a deep dark brown solution will be formed. Similarly,vanadium(III)acetylacetonate will dissolve only to a small degree toform a faintly brown solution, but subsequent to conversion with amagnesium or aluminumalkyl, a dark brown solution will be formed.

The atomic ratio of the element from Group II or III in theorganometallic compound, to the chromium plus vanadium in the 1,3-diketocomplexes, is chosen to be in the range of from about 0.5:1 to 20:1,preferably in the range of from 1:1 to 6:1.

The inert inorganic support is preferably oxide, such as silica,alumina, mixed zirconia, thoria or magnesia. However, preferably silica,alumina, and mixed silica-alumina, and more preferably silica are used.Silica is well known, and may be applicable to the present invention inmany of its different forms. However, silica xerogels, with large porevolumes, are particularly suitable.

Insofar as necessary, the support is dried, for instance, by heating indry air, before the complex transition metal compounds are depositedthereon. The supporting material should be dried, preferably in a mannerand to the extent that the support no longer contains physically boundwater.

The solution of the complexes of the organometallic compound and thechromium and vanadium-1,3-diketo compound is contacted with the inertinorganic supporting material, for instance, by adding the solutionslowly, with stirring, to a suspension of the supporting material in thesame solvent. To the extent the chromium and vanadium compounds are notwholly or partly deposited from the solution onto the support, thisdeposition can be promoted by evaporation of the solvent. The depositionof these compounds from the solution onto the carrier will be clearlypreceptible, in that the solution will become lighter in color, and thesupport will become more darkly colored.

The quantity of the complex chromium compound which is deposited on thesupport in accordance with this invention may be varied within widelimits, but generally should be in the range of between about 0.01 to10% by weight calculated as chromium on the support. It is possible toapply a larger or lesser quantity of the chromium compound, but thisoffers no advantages. Preferably, the amount of the complex chromiumcompound deposited on the support will be such that the chromium contentthereof will be in the range of between 0.02 to 2% by weight, morepreferably in the range of 0.05 to 1% by weight.

The quantities of the complex vanadium compounds deposited on thesupporting material in accordance with this invention, together with thecomplex chromium compound, may also vary within wide limits. Generally,this quantity will be chosen such that the atomic ratio of chromium tovanadium on the supporting material will be in the range of betweenabout 50:1 and 1:50. Preferably, the amount of complex vanadium compounddeposited on the support is chosen so that the chromium:vanadium ratiois in the range of between 20:1 and 1:20, more preferably in the rangeof between 10:1 and 1:10. By varying the atomic ratio of chromium tovanadium on the supporting material, the breadth of the molecular weightdistribution in the resulting polymer can be influenced.

After contacting the complex chromium and vanadium compounds with thesupporting material, the support and metal compounds are separated fromthe solvent by filtration or evaporation. To the extent the complexchromium and vanadium compounds are not deposited on the support to aconsiderable degree, as will be evident from the solvent showing littleor no decolorization the solvent, will be expelled by evaporation.

The resulting support, with the chromium and vanadium compoundsdeposited thereon, is thereafter heated in a non-reducing atmosphere,such as oxygen, air, nitrogen, carbon dioxide or a noble gas at atemperature of between about 200° to 1200° C. Preferably, this heatingis carried out in an oxidizing atmosphere such as, for instance, oxygen,oxygen-containing gases, or air. Air, having a reduced or raised oxygencontent, also forms an oxidizing atmosphere which is suitable as well.

Preferably, the support having the metal compounds deposited thereon isheated at a temperature in the range from 400° to 1200° C., and morepreferably is heated to a temperature within the range of from 500° to1100° C. The heating times may vary from a few seconds to tens of hoursor more. At temperatures of 500° to 1100° C., the heating time willgenerally be in the range of about 30 minutes to 8 hours. The optinumheating time can easily be determined experimentally by preparing, underotherwise similar conditions, catalysts having the same composition, andvarying the heating time at the temperature at which heating iseffected, and by subsequently determining and comparing thepolymerization characteristics of the final catalysts obtained.

Subsequent to the heating step, the supported catalyst (after beingcooled to the ambient temperature) is preferably brought into ahydrocarbon solvent, which preferably is the diluent or medium to beused in the polymerization. This hydrocarbon solvent may be comprised ofaliphatic or cyclic hydrocarbons, such as butane, isobutane, normal orbranched pentanes, hexanes, heptanes, octanes, and higher straightand/or branched saturated aliphatic hydrocarbons, cyclopentane,cyclohexane, cycloheptane, cyclooctane, and the like, or mixturesthereof. Particularly suitable are fractions obtained directly orindirectly from mineral oil, such as light petrol, kerosine, or gasoil,which may contain aromatics, but predominantly consist of aliphaticand/or cycloaliphatics. Aromatic hydrocarbons, such as benzene, toluene,xylenes, or halogenated hydrocarbons can also be used, but for practicalreasons noted above, specifically the high cost and health risksassociated therewith, preference will generally be given to aliphatichydrocarbons or mineral oil fractions containing little or no aromatics.

The invention also includes an improved process for the polymerizationor copolymerization of ethylene using the catalyst described above.

For carrying out the polymerization, the supported catalyst thusprepared is dispersed in an inert diluent, to which an organometalliccompound of an element of Group II or III of the periodic system mayalso be added. Suitable organometallic compounds include those ofberyllium, magnesium, boron, aluminum, or gallium. The molar ratiobetween the organometallic compound added at this point, and thechromium and vanadium components deposited on the support, may varywithin wide limits, for instance, from 0.1:1 to 200:1. Preferably,however, such ratio will be in the range of between 1:1 and 40:1.

The polymerization conditions and the desired characteristics of thefinal polymer determine the extent to which the addition of suchorganometallic compounds is desirable. With low catalyst concentrations,and with relatively low monomer concentrations in the polymerizationmedium or diluent, the addition of such organometallic compounds canoften assist in initiating and sustaining the polymerization. Theimpurities content of the monomer and in the diluent also affects theamount of such organometallic compound to be added. The desired amountof organometallic compound can easily be determined experimentally.

The addition of such organometallic compounds to the polymerizationmedium also slightly increases the activity of the catalyst, and mayslightly reduce the melt index of the polyethylene to be prepared. Theresulting polyethylene obtained with catalysts to which suchorganometallic compounds have been added has a lower density, and oftena broader molecular weight distribution, than polyethylene prepared withthe same catalyst under the same condition and circumstances, butwithout addition of the organometallic component. Thus, when using thecatalyst of the invention, the properties of the polyethylene can bemodified by the addition of organometallic compounds to thepolymerization medium.

Slurry polymerization processes utilizing the catalysts of the inventionare generally carried out at temperatures of at most about 100° C. Withsome volatile solvents, such as, for instance, isobutane, the slurrypolymerization can be carried out at even slightly higher temperatures,up to about 110° C. Preferably, however, the slurry polymerization iscarried out at temperatures of at most about 105° C. The polymerizationcan be carried out at temperatures as low as about 50° C., butpreference is given to selecting a temperature at least 70° C., and morepreferably at least 85° C. or higher.

The polymerization of ethylene can also be carried out using thecatalysts of the invention at temperatures of above 120° C., forinstance at temperatures of from 150° to 200° C. In such instance, asolution is obtained of the ethylene polymer in the solvent. Thissolution can be worked up by methods known in the art. Thepolymerization can also be carried out as a so-called gas phasepolymerization such as disclosed in, for example, U.K. PatentSpecification No. 1,373,982.

The polymerization processes using the catalyst of this invention can becarried out at either atmospheric or elevated pressures. When usinglow-boiling diluents, such as butane, isobutane, or pentane, thepressure in the polymerization reactor will have to be above atmosphericpressure. The monomer pressure may be atmospheric, but preferably willbe higher. By carrying out the polymerization under elevated monomerpressures, higher yields can be obtained. Therefore, generally elevatedmonomer pressures up to for instance 10 MPa will be applied. Even higherpressures, up to for instance 200 MPa or more are possible, butgenerally are not applied for practical reasons.

Preferably, the polymerization will be carried out at pressures betweenabout 600 and 8000 kPa, and more preferably at pressures of between 1200and 5000 kPa. The most desirable pressure will depend, in part, upon thepolymerization temperature and the volatility of the diluent. Theoptimum monomer pressure will be determined, in part, by balancing thehigher polyethylene productivity obtained at higher pressures againstthe increased cost of equipment necessary to operate the polymerizationprocess at higher pressures. The ultimate choice of the optimum monomerpressure is substantially entirely a matter of balancing economicfactors.

The quantity of catalyst present in the diluent or polymerization mediumis generally chosen so that the diluent contains about 0.001 to 10mmoles of chromium, calculated as metal, per liter of diluent, andpreferably between about 0.001 and 0.1 mmoles of chromium per liter.

In carrying out the polymerization using the catalyst, and in accordancewith the process of this invention, various known modifications can beapplied. For instance, molecular weight can be regulated not only bytemperature, but also by the addition of hydrogen or other molecularweight modifiers used for this purpose. Furthermore, the polymerizationcan be carried out in two or more steps, in either a parallelarrangement or in a series arrangement, in which process differentcatalyst compounds, temperatures, residence times, pressures, hydrogenconcentrations and the like can be applied if desired.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments for carrying out the processes of this inventionwill be further elucidated by the following examples. It should beunderstood, however, that these examples are illustrative of some ofmany possible embodiments, and the invention is not limited thereto.

EXAMPLE 1 a. Catalyst Preparation

While being maintained under an atmosphere of dry nitrogen, 2 g ofchromium(III)-acetylacetonate (equivalent to 5.73 mg of chromium metal)is suspended in 80 ml of light petrol having a boiling range of 60°-80°C. A small portion of the chromium(III)-acetylacetonate goes intosolution, as a result of which the solvent is colored faintly violet.Subsequently, 4.33 ml of a 3.97 molar solution of triisobutyl aluminiumin light petrol (equivalent to 17.19 mg of aluminium metal) is addeddropwise while stirring.

Similarly, 2 g of vanadium(III)acetylacetonate (equivalent to 5.74 mg ofvanadium metal) is suspended in light petrol, and 17.19 mmoles oftriisobutyl aluminium was added. Both suspensions are stirred until allchromium and vanadium, respectively, go into solution, whereupon bothsolutions are brought to 100 ml by the addition of further light petrol.

50 g of a silica xerogel with a pore volume of 1.6 cm³ /g and a specificsurface of 250 m² /g is suspended in 250 ml of light petrol.Subsequently, 85 ml of the chromium solution, and 17 ml of the vanadiumsolution are first mixed with one another, and then added, withstirring, dropwise to the suspension of silica xerogel. This resultingmixture is then boiled, under reflux cooling, for one hour whereupon thepetrol is then evaporated. The resulting silica xerogel on which thealuminum trialkyl complexes of chromium(III)acetylacetonate andvanadium(III)acetylacetonate are deposited are then heated for 8 hoursin a stream of dry air at 900° C.

The final silica xerogel supported catalyst contains 0.50% by weight ofchromium and 0.10% by weight of vanadium, calculated as metal.

b. Ethylene Polymerization

An amount of 1500 ml of light petrol and 0.1 g of catalyst areintroduced into a stirred autoclave. 0.2 mmole of trioctylaluminium isadded, and the autoclave contents are heated at 85° C. (resulting in apetrol pressure of 175 kPa), and ethylene is injected until the pressureis 700 kPa. The polymerization is carried out for 90 minutes at 85° C.

A polyethylene product having a melt index (measured according to ASTMD-1238) of 0.01 is obtained. The activity of the catalyst is calculatedto be 1400 g of polyethylene per mg of chromium+vanadium (calculated asmetal) per 100 kPa of ethylene pressure per hour.

EXAMPLES 2-7

In carrying out the polymerization in each of Examples 2-7, 1.2 kg ofisobutane is introduced into a 5 liter autoclave under an atmosphere ofdry nitrogen, whereupon the nitrogen is vented. The autoclave is thenbrought up to the polymerization temperature and ethylene (and hydrogenin Examples 6, 7, and 8) is injected into the autoclave. Approximately400 mg of the catalyst prepared in accordance with Example 1a is addedto the autoclave via a catalyst metering system. In Examples 3-8,triethylboron is added as well. The polymerization is carried out for aperiod of 90 minutes during which polymerization the ethylene pressureis maintained constant.

The conditions, and the results obtained in these Examples 2-7, aresummarized in the Table which follows. On this Table, the third andfourth columns show, respectively, the ethylene and hydrogen pressure inkPa. The fifth column indicates the quantity of triethylboron in mg ofboron per kg of isobutane. The productivity of the particularpolymerization is reported in the sixth column in terms of grams ofpolyethylene produced per gram of catalyst.

It can be seen from Examples 2, 3, and 4 that a definite increase inactivity is obtained by the addition of 1 ppm of triethylboron, but thatgreater additions of triethylboron have no further affect. It can alsobe seen that the addition of triethylboron slightly decreases the meltindex of the resulting polyethylene.

From Examples 3 and 5 it can be seen that raising the polymerizationtemperature from 97° C. to 101° C. increases the melt index from 0.10 to0.21, other conditions remaining the same. From Examples 3 and 7, it canbe seen that the addition of hydrogen, while carrying out thepolymerization at 97° C., results in an increase in the melt index from0.10 to 0.29. The effect of temperature on the melt index of theresulting polymer when carrying out the polymerization in the presenceof hydrogen is shown by a comparison of Examples 6, 7, and 8. From theselatter examples it can be seen that a moderate increase in reactortemperature substantially increases the melt index of the resultingpolymer.

                  TABLE                                                           ______________________________________                                               reactor                                                                       temp.   pressure pressure                                                                             TEB   produc-                                                                              melt                              example                                                                              °C.                                                                            C.sub.2 ═                                                                          H.sub.2                                                                              ppm B tivity index                             ______________________________________                                        2      97      600      0      0     2334   0.11                              3      97      600      0      1     3110   0.10                              4      97      600      0      2     2920   0.08                              5      101     600      0      1     2610   0.21                              6      92      1000     800    1     2515   0.08                              7      97      1000     800    1     2500   0.29                              8      106     1000     800    1     2460   2.8                               ______________________________________                                    

What is claimed is:
 1. An improved process for the preparation of asupported chromium oxide type of catalyst for the polymerization ofolefins wherein the reaction product of a chromium- 1,3-diketo compoundand an organometallic compound of an element from Group II or III of theperiodic system is deposited on an inert inorganic supporting materialwhich is thereafter heated in a nonreducing atmosphere at a temperatureof between 200° and 1200° C., the improvement comprising:reacting, inthe presence of a solvent which is inert with respect to compounds (1),(2), and (3) hereinafter referenced,(1) a chromium chelate of a1,3-diketo compound, and (2) a vanadium chelate or a vanadyl chelate ofa 1,3-diketo compound, separately or jointly with(3) an organo-metalliccompound of an element from Group II or III of the periodic system inwhich hydrocarbyl groups with 1-20 carbon atoms are bound, via a carbonatom, to the element; jointly contacting the resulting solutioncontaining the reaction products of (1) and (2), with (3) with, anddepositing said reaction products on, an inert inorganic supportingmaterial; and heating said supporting material having said reactionproducts deposited thereon in a non-reducing atmosphere at a temperatureof between about 200° and 1200° C.;wherein said 1,3-diketo compounds of(1) and (2) are the same or different and have the formula ##STR4## inwhich formula R₁, R₂, and R₃ are the same or different, R₁ and R₃ beingan alkyl group with 1-10 carbon atoms, and R₂ being selected from thegroup consisting of an alkyl group with 1-10 carbon atoms, and ahydrogen atom.
 2. The process of claim 1 wherein said chelates of said1,3-diketo compounds are reacted with 0.5-20 moles of organometalliccompound of an element from Group II or III of the periodic system permole of chromium plus vanadium.
 3. The process of claim 2 wherein saidchelates of said 1,3-diketo compounds are contacted with from 1 to 6moles of an organo-metallic compound of an element from Group II or IIIof the periodic system per mole of said chromium plus vanadium.
 4. Theprocess of any one of claims 1, 2, or 3 wherein said chelates of said1,3-diketo compounds are reacted with aluminiumtrialkyl.
 5. The processof any one of claims 1, 2, or 3 wherein said chelates of said 1,3-diketocompounds are reacted with a magnesiumdialkyl.
 6. The process of any oneof claims 1, 2, or 3 wherein said chromium chelate of a 1,3-diketocompound is the chromium(III)-acetylacetonate.
 7. The process of any oneof claims 1, 2, or 3 wherein said vanadium chelate of a 1,3-diketocompound is an acetylacetonate.
 8. The process of any one of claims 1,2, or 3 wherein said vanadium chelate of a 1,3-diketo compound isvanadium(III)acetylacetonate.
 9. The process of any one of claims 1, 2,or 3 wherein the molar ratio of vanadium to chromium in said reactionproducts contacted with said inert inorganic supporting material is inthe range of from about 50:1 to 1:50.
 10. The process of claim 9 whereinsaid molar ratio of vanadium to chromium is in the range of between 20:1to 1:20.
 11. The process of claim 9 wherein said molar ratio of vanadiumto chromium is in the range of 10:1 to 1:10.
 12. The process of any oneof claims 1, 2, or 3 wherein said inert inorganic supporting materialsilica.
 13. The process of any one of claims 1, 2, or 3 wherein saidsupporting material having said reaction products deposited thereon isheated in an oxygen-containing atmosphere.
 14. The process of any one ofclaims 1, 2, or 3 wherein said supporting material having said reactionproducts deposited thereon is heated at a temperature of between 400° C.and 1200° C.